Our studies will analyze the thymic peptides FTS (serum thymic factor), thymosin alpha-1, and thymopoietin that induce in vitro differentiation of prothymocytes into T lymphocytes. Synthetic peptides, fragments, and analogs of these peptides will be synthesized, and peptide-protein or peptide-linker conjugates will be prepared and used as immunogens for the production of rabbit antibodies and mouse monoclonal antibodies specific for FTS, thymosin alpha-1, and thymopoietin. Radioimmunoassays (RIAs) for the thymic peptides will be developed using rabbit or monoclonal antibodies and synthetic peptides as the hormone standards and radioiodinated tyrosine analogs as the tracers. These assays will be used to identify the specific antigenic sites recognized by these antibodies and to detect the presence of thymic hormones in human peripheral blood, thymic and peripheral lymphoid tissues, and skin. RIAs will also be used for the quantitation of FTS, thymosin alpha-1, and thymopoietin in human plasma in health and diseases. Analysis of receptors for thymic hormones on certain lymphoid cell populations will be undertaken using fluorescent and radiolabeled derivatives of the thymic peptides. These studies will advance our knowledge of the role of thymic hormones in the normal state and in a variety of immunodeficiency diseases and cancer. Four RIAs have been developed for the quantitation of FTS. Each assay employs an antibody specific for FTS, synthetic FTS as the hormone standard, and a radioiodinated FTS analogue as the tracer. Two assays used the antiserum from a rabbit immunized with an FTS-protein conjugate, and the others used a mouse monoclonal antibody against FTS. One of these RIAs, using the rabbit antiserum, has been optimized to allow quantitation of FTS down to 5 pg. A series of FTS-zinc (thymulin) standards from 1 to 500 pg were quantitated by both RIA and the rosette inhibition bioassay. Good correlation was obtained between these two assays (correlation coefficient, r = 0.873 with p\less\than\0.001). Using both RIA and bioassay, thymulin activity was detected in the supernatants of cultured human thymic epithelial cell monolayers. Its activity could be quantitated between days 8 to 9 to 37 to 50 following initiation of the cultures; secretion was highest between 25 to 40 days. In addition, we have started development of RIAs for thymosin alpha-1. (HF)